In a number of applications of electric power, it is desirable to step down or step up utility ac voltage in order to facilitate effective use of the available power. Such applications include, for example, lamp dimmers and resistive heater controllers. Use of transformers for realizing voltage step-up and/or step-down has been a well known and widespread technology. Transformers may be of an isolated type or of an autotransformer type. However, a drawback associated with the uses of transformers is the large size resulting form large amounts of copper and iron. Further, in order to maintain regulation under varying loads and line voltages, mechanical or thyristor tap changing switches are generally used. In certain cases, the brush contacts of the autotransformer may be motor driven to maintain regulation. However, the performance of such controllers is extremely poor in terms of speed and reliability. In addition, they require extensive periodic maintenance.
Over the past few decades, the use of Silicon Controlled Rectifier (SCR) and triacs has miniaturized the above noted functions through the use of solid state technology. Indeed, the use of SCR and triac ac voltage controllers is widespread in home lamp dimmers and industrial heaters. In such devices, control is exercised by means of variation of a firing angle, which offers smooth and fast response over a wide range. As is known in the art, these devices are generally limited to step down of voltage.
While SCRs and triacs perform the function satisfactorily, they introduce a large amount of harmonics into the utility power lines, as well as to the load, and result in poor power quality. The input power factor also varies widely with the operating range. These power quality problems are compounding as the use of such devices are proliferating in home and industry. Due to their poor waveform quality, they have been primarily used in applications such as heating, where quality does not affect normal operation.
The use of Pulse Width Modulation (PWM) to realize dc-dc power conversion applications is wide spread and well-understood, and thus is not described herein. Moreover, various techniques for realizing single phase ac to single phase ac power control using PWM have been presented in the past. A known single input phase, single output phase, buck converter configuration using ideal switches is illustrated in FIG. 1. Such a configuration is described in FIG. 3 of U.S. Pat. No. 4,347,474, for example. The operation of the circuit is well known and understood. In this configuration, switches X and X' are turned on and off in a complementary manner at a high rate. The output voltage is related to the input voltage through the duty ratio, which is given by the percentage of time that a switch X is conductive ("ON") during the total switching period.
In summary, three phase and single phase ac power conditioning have been realized using tap-changing/autotransformers and thyristor phase control. However, these devices are bulky, slow, and/or suffer from poor waveform quality. With growing concerns of power quality prompted by widespread application of sensitive loads, there is thus a need in the prior art for a new generation of high performance power converters for three phase ac power conditioning, and to realize high performance ac voltage/power control, as well as for efficient circuit structures for AC to DC power conversion.
Moreover, with the prospect of large scale introduction of electric vehicles, there is need for means for transfer of DC power to a vehicle from AC utility sources. In order to maximize the power drawn from the utility, unity power factor operation is highly desirable, as is high frequency operation which results in reduction in the size of transformers used in such a converter.
Known AC-DC converters include SCR configurations, wherein galvanic isolation is effected through the use of a conventional bulky low frequency transformer. Charge rate control is effected through variation of the firing angle of an SCR bridge. Such configurations generally have a low operating power factor. In diode rectifier converters using such transformers, charge rate control is effected through variation of the duty ratio of the converter, resulting in extremely poor power factor. When a high frequency transformer is used in conjunction with a diode rectifier, power factor is improved, but the input current is still rich in harmonics.
In a known boost rectifier configuration of an isolated DC-DC converter, shown schematically in FIG. 2(a), a unity power factor interface is provided to a single phase utility using a boost converter feeding into an intermediate DC bus. The DC bus voltage is subsequently chopped into high frequency AC using resonant or other means, and is passed through a clamp-on or a co-axial type of power transformer, as shown in greater detail in FIG. 2(b). The secondary side of the power transformer contains a rectifier feeding to the load, such as a battery being charged. To achieve high power density of the system, high frequency isolation at the transformer is imperative.
While such an approach fulfills the various power requirements, drawbacks include the requirement for five distinct stages of power transfer in the forward path, shown schematically in FIG. 2(a), each contributing conduction losses and thus reducing the system efficiency. A bulky intermediate DC bus is required to filter out 120 Hz power fluctuations caused by the single phase AC input. Moreover, two controlled power stages are required. To prevent DC voltage overshoots, power balance between input and output stages has to be coordinated, resulting in increased complexity in the control strategy.
There is thus a need in the prior art for highly efficient, unity power factor, AC-DC converters, using a reduced number of stages of power conversion.
There is a more specific need to provide a simplified AC-DC power converter, using only a single power converting stage and a minimum number of components.